1. Field of the Invention
This invention relates to a device and a process for checking the tightness of the covers (generally several) stacked on each other to close the cavity of a radioactive material transport or storage container, for example for irradiated fuels or vitrified residues from reprocessing of these fuels, the said device being used to check the tightness of each of the covers as they are closed in turn, and also subsequently after the container is fully closed, during its life while it is full, during its transport or storage.
2. Discussion of the Background
Radioactive materials, and particularly irradiated nuclear fuel assemblies or residues vitrified from reprocessing, are generally transported and/or stored in heavy thick walled (from a few cm to several tens of cm) containers (also called packagings) with a cylindrical shape and made with one or several layers, mainly based on forged or cast or rolled steel (possibly combined with lead), or based on cast iron which in particular provides functions of mechanical strength (resistance to severe shocks, for example if dropped), radiation shielding and heat transfer
These containers normally include a cylindrical shell closed at one end by a bottom attached in a leakproof manner (for example by welding).
The cavity thus formed, in which the radioactive material is placed, is closed at the other end of the shell, sometimes by a single cover, but usually by at least two removable leakproof metal covers stacked one on top of the other.
One known means of preventing leaks is to use O-rings, either of the elastomer type or metallic type, placed in grooves, the geometry of which must be defined very precisely as a function of the characteristics of the seals to be used. Usually, each cover is fitted with two concentric seals that are in contact with a shoulder formed in the shell.
This seal must be inspectable at all times, or even continuously for containers which are placed in a long term storage location after being loaded.
FIG. 1 schematically shows an example of current practice for creating and checking the leaktightness of a container for nuclear material comprising either a single cover (1), or two superposed covers (1) and (2), or three superposed covers (1), (2) and (3).
A first thick cover (1), or primary cover, is used to confine the radioactive material placed in the container cavity (C).
The cover (1) is in contact with a shoulder formed in the thick metal shell (4), generally cylindrically shaped, forming the container body, by means of two concentric seals (11) located in grooves cut in the cover flange (1), and tightened by means of bolts. It includes a service duct (8) between the cavity (C) and the outside, passing through to the upper surface of the cover (1) through a service orifice. This service duct is used to carry out a number of manipulations in the cavity (C), for example adding or removing water, creating a vacuum, inserting or removing a gas such as He, N.sub.2, etc.
The cover (1) also includes an inspection duct (5) connecting the space between two seals (11) with the outside, passing through to the upper surface of the cover through an inspection orifice onto which various inspection devices (manometers, qualitative and/or quantitative gas analyzer, for example mass spectrometer, vacuum pump, pressurized gas) can be adapted as will be seen later, in order to check the tightness of the seals.
After use, the service orifice (8) is closed by a closing device (not shown) comprising two concentric seals; an inspection take off point, that can be closed by a plug accessible on the top of the cover, opens up between these two seals in order to check their tightness.
The inspection duct (5) is closed by a plug.
Once the primary cover (1) has been installed and its tightness has been checked, the service orifices closed and their tightness checked, a second safety cover (2) or secondary cover, is placed above the first cover (1), using the same methods. Thus this secondary cover comprises two concentric seals (12) in contact with a shoulder formed in the shell, a service duct (9) and the inspection duct (6) used and closed in the same way as for cover (1).
The service duct (9) is used to control the space between the covers (1) and (2) and the inspection duct (6) is used to check the tightness of the seals (12).
The container is ready after closing the cover and checking its tightness, and closing and checking the tightness of the service orifices, and removing the inspection devices connected to orifices (5) and (6). However, once put into storage it is sometime covered by a thick metallic protective top cap (30) to provide better resistance to crashed aircraft.
The following method may be used to check the tightness of the double seals, for example (11) (12):
i) when the cavity (C) is full of a gas, usually helium at 0.5 bars absolute, a vacuum can be created in the space between the seals (11) at a pressure lower than the pressures on each side of the said seals (for example a few mbars), and the pressure rise (if any) in this space can then be observed and measured through the inspection orifice using a manometer of type (14). This method can measure the leakage rate within a range of about 10.sup.-5 to 10.sup.-3 atm.cm.sup.3 /sec. PA1 ii) the space between the seals can be overpressurized with respect to the pressures on each side of the said seals (for example 6 bars), and the pressure drop (if any) can also be measured using a manometer type (14). This method is capable of measuring leakage rates within a range of about 10.sup.-6 and 10.sup.-3 atm.cm.sup.3 /sec. PA1 iii) a helium test can be performed which consists of creating a vacuum in the space between the seals and, when the cavity is filled with helium at pressure P1, measuring the quantity of helium drawn in through a leak in the seal (if any) using a mass spectrometer previously calibrated using a calibrated leak. This method is much more sensitive and can detect leaks of between 10.sup.-9 and 10.sup.-6 atm.cm.sup.3 /sec.
By using different gases on each side of the seals, it is possible to determine which seal (inside or outside) is leaking.
Thus, when the cover (1) is put into place and after filling the cavity (C) with gas at a pressure P1 less than atmospheric pressure (usually helium at 0.5 bars absolute, as mentioned above), the tightness of the double seals (11) can be tested, and then the tightness of the double seals of the closing device in the service orifice(s) can be tested using the test take off points leading into the space between the seals.
When these verifications have been terminated, the cover (2) is put into position, and the space between the covers (1) and (2) is filled with a gas at pressure P2 usually greater than P1 (typically helium or nitrogen with a working pressure of 6 bars) and checks on the tightness of the various seals can be made as for the cover (1).
The pressure P2 can be continuously monitored using a pressure sensor. If this pressure reduces after long term storage of the container, then there must be a leak either to the atmosphere or into the container cavity (C) since the pressure P2 is significantly higher than the outside atmospheric pressure, and obviously than the lower pressure P1 in cavity (C). It can thus be seen that radioactivity is confined and that it is impossible for this radioactivity to be released from the container cavity to the environment.
In order to take appropriate corrective action, the origin of the leak must be determined by checking the tightness of each of the covers (1) and (2).
The first step in doing this is to remove the protection cover (30) to provide access to the take off point used to check closure of the service orifice (9) and the inspection orifice (6) of the seals (12) to check their tightness.
If it is found that the seal is tight, it is deduced that the leak is located in the primary cover, which for example makes it impossible to remove the cover (2) to eliminate any risk of disseminating radioactivity into the atmosphere.
However, if it is found that there is a leak at one of the previously tested seals, then it remains to be determined if this leak is sufficient to explain the observed pressure drop before concluding that there is no other leak on the cover (1).
When a leak is detected on the primary cover (1) of the container, the solution usually adopted consists of putting a closing cover (3), equipped like covers (1) and (2) with two concentric seals (13) in contact with a shoulder on the shell, with an inspection orifice (7) for checking the tightness of the concentric seals (13) on the cover (3) and a service orifice (10) designed to create a gas pressure P3 between covers (2) and (3), this orifice also being closed by a closure with double inspectable seals.
It can be seen that with this type of device, it is difficult to isolate the location of the leak on the covers with certainty, and particularly to observe a leak on the primary cover directly and consequently to provide an appropriate remedy.
Furthermore, before these inspections can be carried out, the heavy protective top cap (30) has to be removed firstly as already mentioned in order to access the various service or inspection orifice plugs.
Thus, the applicant attempted to search for a device and a process in order to locate any leaks on each of the covers, independently of each other, both on the main seals and on the different working orifice seals while simplifying tightness inspection operations, particularly during storage of the container.